With Cage becoming more popular since its mention on Phoronix and
therefore getting more use-cases than just my own project, add XWayland
support. The refactoring of 2cf40f7 makes this much easier. Note that
this is a no-cost addition for those of us not using XWayland as it is a
compile-time option that needs to be explicitly enabled by adding
`-Dxwayland=true` to your meson command.
We shouldn't render a window before it is mapped (obviously), but we
render all windows in the view list. Hence, only insert the window once
it is mapped.
We could run into the case where a window is destroyed without being in
the window list, so we now track unmapping again and remove windows from
the list when they get unmapped.
This makes Cage much easier to maintain. Not only is it easier where to
look and to maintain a mental model of the code, there is also more
encapsulation, better abstractions and better extendability.
